Zimbabwe-Rhodesian troops and warplanes hammered regular Mozambique Army targets in the southern part of that country today, the second day of an incursion that the military headquarters here called "a major self-defense operation."

The attack came four days before a British-sponsored constitutional conference on Zimbabewe-Rhodesia in London. It was the deepest such incursion into Mozambique and marked the first time that Zimbabwe-Rhodesian forces deliberately struck at Mozambique Army targets.

Observers interpreted the timing of the raid as a major show of strength by Salisbury as the London conference approaches.

Prime Minister Abel Muzorewa, his predecessor Ian Smith and 10 other delegates of the Salisbury government are to attend the conference along with Joshua Knomo and Orbert Mugabe, co-leaders of the Patriotic Front guerrilla forces fighting Zimbabwe-Rhodesia. Mugabe's Mozabique-based forces came under attack in the latest raid.

In London, Washington Post correspondent Jay Ross reported that the raid, which sharply escalates the level of warfare by bringing Mozambican forces into the fighting, could signal an attempt by the Salisbury government to split Mugabe and Nkomo on the eve of the London conference.

There have been recent reports that Mozambican forces accompanied Mugabe's guerrillas on missions into Zimbabwe-Rhodesia, but no evidence of any direct involvement by Mozambique in the fighting.

Britain has called for a cease-fire before the London negotiations begin next week, but the Patriotic Front guerrillas have refused, saying they will agree to a truce only when all arrangements have been worked out.

The conference is aimed at ending Zimbabwe-Rhodesia's seven-year guerilla war, in which black nationalist guerrillas have been fighting to take power from a white minority that still retains considerable influence and privileges in Zimbabwe-Rhodesia despite a transition to a government at least nominally headed by moderate blacks.

The communique of the military headquarters in Salisbury justified the latest attacks by saying that the Mozambiqu Army had taken over logistics and protection of Mugabe's Zimbabwe African National Liberation Army (ZANLA) in the southern Gaza Province.

The communique said the operation was intended to pre-empt a buildup of equipment and weapons which was "far in excess of purely defensive military requirements and confirms the growing threat of [Mozambique Army] incursions into the southeastern border area of Zimbabwe-Rhodesia."

Last week Muzorewa charged that Mozambique had committed regular troops to fight alongside the guerrillas inside Rhodesia.

The communique said the raiders yesterday knocked out the supply lines of joint Mozambique-ZANLA forces in the Aldeia da Barragem area on the Limpopo River about 200 miles inside Mozambique. Today, it said, the troops attacked the main Mozambique-ZANLA brigade headquarters at Mapai about 55 miles from the border.

The statement added that the attackers killed 23 of 26-man army unit in a fierce gunbattle and destroyed a fuel dump, a radar station and an armory as well as five road and rail bridges.

The headquarters said one of the invaders was killed and three wounded, but did not say how many were sent. The Rhodesian died when his U.S. built Bell 205 helicopter, one of an undisclosed number obtained recently despite international trade sanctions, was shot down.

The Mozambique Defense Ministry said the attack began with French-built Mirage fighters escorting helicopter-borne troops across the border. The reference to Mirages was apparently intended to suggest South African air support for the raid, since Zimbabwe-Rhodesia has no Mirage fighters but uses vintage British Hawker Hunters.

South Africa has consistently denied any involvement in previous Zimbabwe-Rhodesian raids into Mozambique, Zambia and Angola. Unlike the latest attack, those raids were aimed primarily at black nationalist guerrilla bases in those countries.

Most cross-border attacks in recent months have been aimed at guerrilla forces in Zambia to the north. The worst single death toll during the war resulted from a raid into Mozambique in late 1977 when an estimated 3,000 guerrillas and their sympathizers were reported killed.